A diesel runaway is not someone legging it off the forecourt with a tank full of the heavy oil, but if you’ve experienced the phenomenon you will know the term and what a frightening situation it can be.
The term “runaway” is used when a diesel engine is self-fuelling by running on is own engine oil and revving out of control. Without any intervention, the final result is usually self destruction or worse case scenario explosion.
Many people who know about the phenomenon incorrectly assume that it is only the older mechanical diesel engines that are prone to this situation, this is not true.
There are three things that are needed for the combustion engine to run, air, fuel and heat. Diesel engines produce their own heat due to the high compression ratio. The airflow is unrestricted on a diesel and the engine is regulated only with fuel delivery. In a runaway situation, something usually causes the engine to start sucking in its own oil. This could be a broken piston ring, turbocharger seal fail, head gasket and several other possible causes. As the engine oil is sucked in, the engine starts to run off of that oil, once that happens, the engine is supplying its own heat, fuel and air is drawn in as required.
There are now only two ways the engine will stop.
A, the engine will run until it self-destructs.
B, remove the air supply. This is way easier said than done. Simply turning off the ignition or disconnecting the battery, will have no effect at all, the engine is now self-propelling.
Without a mechanical shut off valve, it becomes extremely difficult to cut off all of the air. Your only option is to disconnect an air intake hose and cover the end with a solid flat surface such as a book or piece of wood (never use your hand), however, if there is a small gap, the engine could continue to run, just at a much lower speed.
If you recognise the symptoms quickly enough you may be able to react and save the engine with the appropriate action. The time scale for intervention is usually single figure minutes before destruction of your vehicle’s power-plant. If you are unfamiliar with the engine set up, it is advised to evacuate the vehicle and stay well clear, the engine could explode showering hot pieces of metal around the engine bay.